1. Field of the Invention
This invention relates to a computed tomographic method, and more particularly to a computed tomographic method in which a plurality of X-ray projection distribution images obtained by exposing a section of an object to X-rays in many different directions are once stored in a stimulable phosphor sheet, and the stored image information is then scanned with stimulating rays and converted into electric signals to compose a tomographic image of the object.
2. Description of the Prior Art
In a computed tomographic apparatus (hereinafter referred to as a CT apparatus) developed by Hounsfield et al., a tomographic image of a predetermined section of an object is composed from a plurality of X-ray projection distribution images obtained by exposing the section of the object to X-rays in many different directions. The CT apparatus can provide a clear tomographic image of a soft tissue, which could not be obtained with the conventional method using X-ray films, and attracted attention in the field of medical diagnosis.
FIG. 1 shows a typical example of the CT apparatus used at present.
As shown in FIG. 1, the CT apparatus has a tomographic unit comprising an X-ray tube 3 for emitting X-rays 2 of a fan-like distribution (normally spreading at an angle in the range between 30.degree. and 40.degree.) covering a tomographic region 1 where an object is positioned, and a detector 4 for detecting the X-rays 2 transmitting through the object. The detector 4 consists of several hundreds of detecting elements densely positioned to form and arc at an equal distance from the X-ray tube 3. As the detecting elements there are used photomultipliers provided with scintillators on light receiving surfaces, high-pressure xenon gas detecting elements, semiconductor elements, or the like. The X-ray tube 3 and the detector 4 can integrally rotate around the object in the directions of the arrows 5 and 6. The CT apparatus is also provided with a computer for processing the X-ray projection distribution images obtained by detecting the X-rays transmitting through the object by the detector 4.
When taking a tomogram, the X-ray tube 3 and the detector 4 are set at an angle with respect to the object, and X-rays 2 are emitted pulse-wise from the X-ray tube 3. The X-rays 2 transmitting through the object are detected as an X-ray projection distribution image by the detector 4. The X-ray projection distribution image detected is stored in a memory of the computer. After the tomographic operation is finished for one angle with respect to the object, the X-ray tube 3 and the detector 4 are rotated by a small predetermined angle with respect to the object in the directions of the arrows 5 and 6, respectively. The tomographic operation is again conducted as described above to obtain another X-ray projection distribution image, which is then stored in the memory of the computer. The operations mentioned above are repeated to sequentially obtain the X-ray projection distribution images at various predetermined angles with respect to the object. In this way, the operations for obtaining a tomographic image of the object is finished when the X-ray tube 3 and the detector 4 have been rotated one turn (360.degree.) around the object. Then, the information of the X-ray projection distribution images obtained at various angles with respect to the object and stored in the memory of the computer is computed and processed by the computer to obtain a tomographic image of a section of the object.
In another type of CT apparatus, an X-ray tube and a detector consisting of a relatively small number of detecting elements are used to linearly scan an object at an angle with respect to the object. After conducting the linear scanning, the X-ray tube and the detector are rotated by a small predetermined angle with respect to the object, and linear scanning is conducted again. Thereafter, the rotation of the X-ray tube and the detector and the linear scanning are repeated alternately until the tomographic operations are finished when the X-ray tube and the detector have been rotated 180.degree. with respect to the object. There is also a CT apparatus in which detecting elements are positioned in a ring-like from extending 360.degree. around an object, and only the X-ray tube is rotated 360.degree..
In general, to achieve high diagnostic accuracy and efficiency, the tomographic image should exhibit a spatial resolution sufficient to permit discrimination of details of a tissue. In the conventional CT apparatus described above, the spatial resolution of the tomographic images finally obtained depends on the number of detecting elements of the detector per unit space. However, when photomultipliers provided with scintillators on light receiveing surfaces are used as the detecting elements, a sufficient spatial resolution cannot be obtained because such photomultipliers have a large size and the number thereof per unit space of the detector becomes small. High-pressure xenon gas detecting elements and semiconductor detecting elements can be positioned more densely than the photomultipliers. In this case, however, the available spatial resolution is at most about 1 line/mm. Furthermore, high-pressure xenon gas and semiconductor detecting elements exhibit a lower sensitivity than the photomultipliers provided with scintillators on light receiving surfaces. Therefore, to obtain a tomographic image of the same quality as when using photomultipliers, it is necessary to increase the dose which the object receives. Further, when the detector is formed of a plurality of detecting elements, the respective detecting elements should exhibit the same sensitivity. However, it is technically very difficult to make the detecting elements exhibit exactly the same sensitivity.
Furthermore, in the conventional CT apparatus described above, at least the X-ray tube is intermittently rotated by predetermined angles around the object, and once stopped at the respective angles to apply X-rays to the object and obtain an X-ray projection distribution image. In this case, the time required for the X-ray tube to be moved to change the angle of X-rays with respect to the object constitutes a dead time in the tomographic operation, prolonging the cycle time of tomography. However, an increase in the tomographic time presents a very real problem in cases other than when recording a stationary image of a head, a skeleton, or the like. Namely, when recording an image of an internal organ, the condition of the organ changes due to muscular motion, breathing motion, vermicular motion, or the like during a prolonged tomographic time. As a result, the contrast and the spatial resolution of the tomographic image finally obtained are adversely affected, or noise called artifact occurs, making it impossible to obtain a tomographic image suitable for viewing and diagnostic purposes.